I need to power a gadget I am building with 5 x AA batteries (6V), but I also want to be able to recharge it via USB (5V) while the gadget is on (that is, I want to be able to plug and unplug the USB cord freely, without the need to remove the battery or turn the device off).

I have read a lot already about this, but there seem to be so many options to choose from that I am rather confused.

So the core of my question is:

Is there any well-known off-the-shelf component or IC to boost 5V from USB to (more than) 6V so I can charge the pack?

As an example of what I understand as "well known off the shelf component", I could mention "4017" as counter, "555" as timer/oscillator, "7806" as voltage regulator, etc. It's something I could find and use easily and cheaply.

  • \$\begingroup\$ You may find it easier in the long run to switch to using Li-ion 18650s instead. \$\endgroup\$ – Ignacio Vazquez-Abrams Apr 14 '15 at 15:15
  • \$\begingroup\$ @IgnacioVazquez-Abrams Took a look and that would surely be an option, since I would benefit from a little more than 6V, to be able to power two bright leds in series (impossible with 5V). So what would you recommend then? \$\endgroup\$ – heltonbiker Apr 14 '15 at 16:05

Based on the comments in the question, if you want to use two AA Li-Ion cells, charged from 5V USB input, you will need to boost the 5V up to 8.4V because each of the cells will need 4.2V charge voltage. Furthermore, you will need a special charge management chip; you can't just run 8.4V across the two cells.

There are far fewer charge management chips for two cells that for a single cell, the MCP73213 is one. There is a wider range of boost regulators that will take 5V in and can be configured to output 8.4V out; I chose the LMR62014.

Note that the maximum input you can expect from a USB port is 1A; according to the datasheet you will only get about 600 mA out from the boost regulator. So charging 2 AA Li-Ion 2600 mA cells to full capacity at 0.2C will take some time.

enter image description here

For NiCd or NiMH batteries, you can use essentially the same circuit, substituting a suitable charge management IC for the MCP73213 for those chemistries such as the LT1511.

  • \$\begingroup\$ Nice answer: thorough, simple, ad to the point! Thanks!! \$\endgroup\$ – heltonbiker Apr 14 '15 at 19:27
  • \$\begingroup\$ I have never seen a Lithium Ion AA rechargeable cell. Got a link? \$\endgroup\$ – mkeith Apr 14 '15 at 19:29
  • \$\begingroup\$ @mkeith here you go mate! amazon.com/UltraFire-1200mAh-Rechargable-Batteries-4-count/dp/… \$\endgroup\$ – KyranF Apr 14 '15 at 19:33
  • \$\begingroup\$ I have some TrustFire brand ones, but their little protection circuit on top of the end means it rarely fits into a AA holder properly :( \$\endgroup\$ – KyranF Apr 14 '15 at 19:34
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    \$\begingroup\$ @mkeith I was going by the battery mentioned by Ignacio Vazquez-Abrams in his comment in the answer, a Li-ion 18650. \$\endgroup\$ – tcrosley Apr 14 '15 at 19:39

There is not enough total power available from a USB port to charge 5 AA batteries. A PC may only provide 500 mA at 5V. That is 2.5W. 5 NiMH batteries would be around 12 Wh. Ignoring losses, 12Wh / 2.5W = 4.8 h. But charge termination for NiMh batteries is problematic unless the charge rate is at least 0.33C (3 hour charge rate). You could trickle charge, but NiMh batteries aren't as robust when trickle charged as the older NiCd batteries were. It will work, but it will cause the battery endurance to be less than specified.

Some have suggested lithium batteries. That might work better. Lithium batteries use a different method to decide when to terminate charge, and it works at slower charge rates. However, you will need to manage the power path if you want to charge while the device is powered. In order for the lithium charger to work correctly, the load must not be in parallel with the battery. The charger needs to know the actual charge current into the battery to terminate correctly. You can google "power path charger" to read up on this.

  • \$\begingroup\$ This "power path" thing is something I'm very glad you told me! Now an additional question: do you any specific power-path charger that works with two cells (7.4V)? \$\endgroup\$ – heltonbiker Apr 14 '15 at 20:17
  • \$\begingroup\$ No. TI probably has over 100 charger IC's for Lithium batteries. You can take a look there. I am not sure whether they have many multi-cell charger IC's. \$\endgroup\$ – mkeith Apr 14 '15 at 21:01
  • \$\begingroup\$ Looks like BQ24133, from TI, meets the requirements. And it also seems to work from USB (Vin > 4.5V). \$\endgroup\$ – heltonbiker Apr 14 '15 at 21:33
  • \$\begingroup\$ That is a very nice and very fancy charger. ;-) \$\endgroup\$ – mkeith Apr 16 '15 at 20:00
  • \$\begingroup\$ Do you think this means it's also (too) expensive? :o (Well, not quite, TI tells it's under 5 bucks). \$\endgroup\$ – heltonbiker Apr 16 '15 at 21:01

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